Multiline strings

[pcwalton]

We should support multiline strings, possibly both by backslashing escapes and with Python-style triple-quoted strings.

[brson]

Strings can already span multiple lines. rustc does this in driver::usage().

[pcwalton]

Well, what I mean is that we should be able to make new lines by putting \ at the end of the line and having it eat the newline (and possibly all the whitespace following it).

[pcwalton]

Fixed by jdm in a078844!